1. Field of the Invention
This invention relates to an optical semiconductor device such as a quantum well laser, a quantum well light-modulator, a quantum well optical-waveguiding circuit, etc., which contains a quantum well optical-waveguiding region, utilizing a quantum effect within semiconductor thin films formed by molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc.
2. Description of the Prior Art
Recently, a single crystal growth technique for the formation of thin films such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc., has been developed which enables the formation of thin film growth layers having a thickness of as thin as appoximately 10 .ANG.. The development of such a technique, although these significantly thin films have not yet been produced by liquid phase epitaxy (LPE), allowed the thin films to be applied to lasers, resulting in laser devices exhibiting new laser effects. A typical example of these new laser devices is a quantum well (QW) laser, which is produced based on the fact that quantization levels are established in its active layer by reducing the thickness of the active layer from several hundred .ANG. to approximately 100 .ANG. or less and which is advantageous over conventional double-heterostructure lasers in that the threshold current level is low and the temperature and transient characteristics are superior. Such a quantum well laser is described in detail in the following papers:
(1) W. T. Tsang, Applied Physics Letters, vol. 39, No. 10, pp. 786 (1981) PA0 (2) N. K. Dutta, Journal of Applied Physics, vol. 53, No. 11, pp. 7211 (1982), and PA0 (3) H. Iwamura, T. Saku, T. Ishibashi, K. Otuka, Y. Horikoshi, Electronics Letters, vol. 19, No. 5, pp. 780 (1983).
As mentioned above, a single crystal growth technique, such as molecular beam epitaxy or metal-organic chemical vapor deposition, has resulted in the practical use of high quality semiconductor lasers having a new multiple-layered structure.
FIG. 2 shows a conventional laminated structure of a single quantum well laser, having carrier reservoir layers, which is made by the successive growth of an n-GaAs buffer layer 2 having a thickness of 0.5 .mu.m, an n-Ga.sub.0.5 Al.sub.0.5 As cladding layer 3 having a thickness of 1 .mu.m, a non-doped Ga.sub.0.7 Al.sub.0.3 As carrier reservoir layer 4 having a thickness of 0.1 .mu.m, a non-doped GaAs quantum well active layer 5 having a thickness of 0.01 .mu.m, a non-doped Ga.sub.0.7 Al.sub.0.3 As carrier reservoir layer 6 having a thickness of 0.1 .mu.m, a p-Ga.sub.0.5 Al.sub.0.5 As cladding layer 7 having a thickness of 1 .mu.m and a p-GaAs cap layer 8 having a thickness of 0.3 .mu.m on an n-GaAs substrate 1 by molecular beam epitaxy. FIG. 3 shows the distribution of the AlAs mole fraction (i.e., x) in a Ga.sub.1-x Al.sub.x As mixed crystal in the laminated structure in FIG. 2, which is composed of the cladding layers 3 and 7, the carrier reservoir layers 4 and 6, and the active layer 5. Laser oscillation is achieved in a GaAs quantum well active layer 5 having the low AlAs mole fraction (i.e., x) in a Ga.sub.1-x Al.sub.x As mixed crystal. This active layer 5 has a thickness of 0.01 .mu.m which is optically thin enough compared with the wavelength of light, so that its existence is optically negligible. Thus, the waveguiding of light therein is carried out by an optical waveguide which is formed by the carrier reservoir layers 4 and 6 and the cladding layers 3 and 7. The laser oscillation in the above-mentioned laser structure is attained depending upon the quantization levels which are established in the single quantum well active layer 5. The active layer 5 is extremely thin and sandwiched between the carrier reservoir layers 4 and 6 having a great energy gap therebetween. The quantization levels are produced due to steep changes in the energy gap at the interface between the quantum well growth layers, and thus composition of the active layer 5 is required to be dramatically different from that of each of the carrier reservoir layers 4 and 6. However, since the composition of the interface between the quantum well growth layers in the conventional laser structure shown in FIG. 2 is a GaAlAs ternary mixed crystal, microscopical arrangements of Ga and Al in a direction which is parallel to the interface between the growth layers are not uniform and/or irregular so that steep changes in the energy gap at the interface between the growth layers cannot be attained and swayed changes therein are created. This is the reason why ideal quantum well characteristics have not yet been attained in the quantum well structure.
As the quantum well structure, in addition to the single quantum well mentioned above, a multi-quantum well having the AlAs mole fraction (i.e., x) in a Ga.sub.1-x Al.sub.x As mixed crystal as shown in FIG. 4 has been used, which is made in such a manner that a multi-quantum well structure which is composed of alternate layers consisting of five GaAs quantum well active layers (the thickness of each layer being 0.01 .mu.m) 12 and four Ga.sub.0.7 Al.sub.0.3 As barrier layers (the thickness of each layer being 0.005 .mu.m) 13, is sandwiched between Ga.sub.0.7 Al.sub.0.3 As cladding layers 11 and 11. According to the multi-quantum well structure, the average AlAs mole fraction (i.e., x) in a Ga.sub.1-x Al.sub.x As mixed crystal of the quantum well consisting of the active layer 12 and the barrier layer 13 is approximately 0.086, which is smaller than that of Ga.sub.0.7 Al.sub.0.3 As, and moreover the thickness of the quantum well region is as thick as 0.07 .mu.m, so that the multi-quantum well structure can serve as an optical waveguide. Moreover, an increase in the number of both the quantum well active layers 12 and the barrier layers 13 allows an increase in the proportion of light to be waveguided in the quantum well. Thus, the application of a multi-quantum well to an optical waveguiding device (such as an optical waveguide, an optical switch, a light modulator, etc.,) employing the Exitonic efect which is specific to quantum well structures is advantageous over that of a single quantum well thereto. However, if such ternary mixed crystal as GaAlAs were used for the barrier layers 13 and/or the cladding layers 11 in the multi-quantum well structure, the swayed changes in the energy gap at the interface between the growth layers would have a decremental influence upon the resulting multi-quantum well optical waveguide.